Spin-inversion and spin-selection in the reactions FeO⁺ + H₂ and Fe⁺ + N₂O

Abstract

The reactions of FeO(+) with H2 and of Fe(+) with N2O were studied with respect to the production and reactivity of electronically excited (4)Fe(+) cations. The reaction of electronic ground state (6)FeO(+) with H2 was found to predominantly produce electronically excited (4)Fe(+) as opposed to electronic ground state (6)Fe(+) corresponding to a spin-allowed reaction. (4)Fe(+) was observed to react with N2O with a rate constant of 2.3 (+0.3/-0.8) × 10(-11) cm(3) molecule(-1) s(-1), smaller than the ground state (6)Fe(+) rate constant of 3.2 (±0.5) × 10(-11) cm(3) molecule(-1) s(-1) (at room temperature). While the overall reaction of (6)FeO(+) with H2 within the Two-State-Reactivity concept is governed by efficient sextet-quartet spin-inversion in the initial reaction complex, the observation of predominant (4)Fe(+) production in the reaction is attributed to a much less efficient quartet-sextet back-inversion in the final reaction complex. Average spin-inversion probabilities are estimated by statistical modeling of spin-inversion processes and related to the properties of spin-orbit coupling along the reaction coordinate. The reaction of FeO(+) with H2 served as a source for (4)Fe(+), subsequently reacting with N2O. The measured rate constant has allowed for a more detailed understanding of the ground state (6)Fe(+) reaction with N2O, leading to a significantly improved statistical modeling of the previously measured temperature dependence of the reaction. In particular, evidence for the participation of electronically excited states of the reaction complex was found. Deexcitation of (4)Fe(+) by He was found to be slow, with a rate constant <3 × 10(-14) cm(3) molecule(-1) s(-1).

Automated reaction path searches for spin‐forbidden reactions

“…S1 of Supporting Information). In addition to this energetic uncertainty of the B3LYP method, it may be important to mention that a recent experimental study has shown that the reaction of FeO + with H 2 in the gas phase mainly produces the electronically excited 4 Fe + cation . This suggests a much less efficient quartet–sextet transition in the product complex region presumably due to a small spin–orbit coupling …”

Two‐State Reactivity: Personal Recounting of its Conception and Future Prospects

“…1). Recent and much more sophisticated studies, [27] concluded that most likely the reaction efficiency is limited by the 4 TS species ( Figure 5), which acts as a bottleneck due to sparser density of states than in the entrance channel (in simple terms, 4 TS is an "entropic" bottleneck). While this may indeed be the case for the specific reaction of FeO + with H 2 , it is not necessarily applicable to all other reactions.…”

“…Despite the impossibility of calculating the P450 reactions at any reasonable level, Helmut, like Old Kato, would repeat his questions on the controversy whenever we met, be it in an Italian restaurant or in a concert… I spent many hours thinking including dreaming in my sleep about the electronic structure of the active species of P450, so called Compound I (Cpd I). After a short seminar tour in Germany during October [22][23][24][25][26][27]1996, I began to understand and formulate the electronic structure of Cpd I using both VB and MO formulations. Thus, as shown in Figure 6, since Cpd I is neutral, and since the porphyrin (Por) and the oxo ligand have each an oxidation number À 2, and the thiolate (cysteinate) ligand is À 1, the effective oxidation state of iron is Fe V .…”

Collisions of FeO+ with H2 and He in a Cryogenic Ion Trap

“…However,t he growth of the larger complexes by sequential addition of He would requirea nomalously huge ternary rate coefficients for bridging the gap towards n = 5. One solutioni st hat they are formed from 4 Fe + .S pin relaxation of quartet ions in collisions with He is probably slow enough (some 10 À14 cm 3 s À1 [41] )t hat some of them form complexes with helium. The energy of He- 4 Fe + is lower than that of He-6 Fe + ,a nd therefore spin isomerization should no longero ccur.T he addition of more He is very Fe + with n greater than 8.…”

“…The situation of adding He to Fe + becomes more complicated if the ions are formed inside the trap by reaction (1). In this case most Fe + products are in the quartet state [41] and result in different mass spectra containing signals for up to 10 %o f (H 2 ) n -Fe + ions with am aximum at n = 4. In general, however, we use in the following experiments such low hydrogen densities that secondary reactions of Fe + ions can be safely ignored.…”